The compound 2,6-diaminopyridine (“DAP”), which is shown by the structural formula below
is a useful starting material for preparing monomers for rigid rod polymers such as described in WO 94/25506, as well as for dyes, metal ligands, medicines and pesticides.
It is well-known to prepare DAP by means of the Chichibabin amination reaction in which pyridine is reacted with sodium amide in an organic solvent. This is a complicated reaction requiring relatively severe conditions (e.g. 200° C. at elevated pressure). Additionally, handling sodium amide and isolating the desired product from this complex mixture are difficult operations to perform on a commercial scale.
Pyridine derivatives have also been synthesized through the conversion of acylic dinitriles (see, for example, GB 2,165,844; and U.S. Pat. Nos. 5,066,809; 4,876,348; 6,118,003; 4,603,207; 5,028,713; and 4,051,140). There are several examples of this type of transformation in the vapor phase and, to a lesser extent, in the liquid phase. For example, U.S. Pat. No. 4,876,348 teaches the ammoxidation of 2-methylglutaronitrile to 3-cyanopyridine. The two-step process involves the conversion of 2-methylglutaronitrile to a mixture of 3-methylpyridine and 3-methylpiperidine, followed by the vapor phase ammoxidation of this mixture with NH3 and O2 in the presence of a complex metal oxide to give the product 3-cyanopyridine.
A need thus remains for a process wherein, on an industrial scale, it would be possible to form pyridine derivatives in the liquid phase in the presence of a heterogeneous catalyst, which would facilitate separating and recycling the catalyst.